Innovative picture reconstruction and analysis methods filter out cosmic rays in order to find elusive neutrinos.
The Science Physicists have devised new methods to assist reduce cosmic “noise” while looking for indications of neutrinos in detectors near the Earth’s surface. This technology combines data-sifting techniques with image reconstruction methods similar to those used in medical computed tomography (CT) scans. It distinguishes neutrino signals generated by particle accelerators from the “web” of tracks created by cosmic rays. These cosmic visitors outnumber neutrino interactions in the detector by a factor of 20,000. Filtering out the many trails left by cosmic rays might assist research on the Earth’s surface aimed at understanding the behavior of subatomic neutrinos.
The new approach was created by scientists for the MicroBooNE detector. MicroBooNE is one of three detectors at Fermilab that are detecting neutrinos generated by an accelerator. Scientists are attempting to understand how these subatomic particles switch between three “flavors” by measuring the quantity of each kind at various distances from the accelerator. Detecting differences in neutrino counts might aid scientists in understanding the process of flavor-shifting, known as oscillation. It might potentially point to the presence of a fourth neutrino type. The method should be applicable to all surface-based neutrino detectors where cosmic rays might obscure signals, and it will help the whole US neutrino research program.
At the Earth’s surface, neutrino detectors must separate the signals of elusive neutrino interactions from the background “noise” of cosmic rays. MicroBooNE recognizes tracks created when charged particles from neutrino interactions ionize the detector’s argon atoms. In the MicroBooNE experiment, three planes of wires are sensitive to electrons in the ionization trails. Each plane records a two-dimensional picture of the railway. Computers build 3D pictures of internal organs from 2D “slice-like” photographs of the human body, similar to how computed tomography (CT) scanners produce 3D images of internal organs from 2D “slice-like” snapshots of the human body. The geometry of the track at MicroBooNE informs scientists which flavor of neutrino sparked the encounter. To sift out millions of tracks caused by cosmic rays, scientists first compare the track signals to light flashes caused by neutrino interactions. The researchers used algorithms to compare the timing and light patterns of each photomultiplier tube in the detector to the positions of all of the particles’ tracks. If there is no match, it is not a neutrino event. They also devised ways for removing tracks that fully cross the detector and detecting tracks that originate inside the detector rather than outside, completing the task of zeroing in on neutrino occurrences.
The Department of Energy Office of Science, Office of High Energy Physics, supported this research. The Fermilab Accelerator Complex, which generates neutrinos for MicroBooNE and other short-baseline neutrino investigations, is a user facility of the DOE Office of Science.
MicroBooNE experiment provided the image.
Electron-neutrino interaction traces (black) linked with accompanying light signals from photomultiplier tubes (red circles) stand out clearly among the countless tracks created by cosmic rays in this MicroBooNE data (dimly colored lines)